Vern, Typically a BP/BR has a wide pass and narrow notch. On mobile duplexers the pass is usually very broad. Base type often have pretty good attenuation a few MHz away. Adding a bandpass will improve this.
No cavity is going to illimate something 16 kHz away....well and let the desired sig thru. On many duplexers a notch might be 50-100 kHz wide, but the pass is MHz wide. For 16 kHz one would have to rely on the receiver selective IF filters, but for typical FM NB repeaters this is a bit much. For SSB no problem assuming the signal is not very strong. 73, ron, n9ee/r --- In [email protected], "w6nct" <[EMAIL PROTECTED]> wrote: > > I'm tuning up a cavity-type duplexer for a 70cm repeater, and in the > process, I showed my results to a fellow ham. He asked a couple > questions that cause me to think; so I decided to ask the experts... > > FIRST QUESTION TOPIC: > > For reference, my primary cavities were made by Tx-Rx Systems, and > have both bandpass and band-reject tuning controls on each cavity. > When tuning them (either singularly or as pairs in-series), I see and > can adjust the band-pass and notch for the desired Tx and Rx frequencies. > > However, in the broad-band sweep, I can also see a bunch of other > signals passing through the cavities; all of which are well away from > my Tx/Rx frequencies. I suspect that these are normal, and are a side > effect of how the can-type resonant cavities work. Am I correct in > this assumption? > > As a sanity check, I combined this set of cavities with another > (Phelps-Dodge) set I have, and tuned these supplemental cavities for > band-pass only (one cavity for my Tx frequency, and one cavity for my > Rx frequency). Sure enough when I put these in series with my primary > set, I can eliminate nearly all of the "other signals" from the > broad-band sweep. This observation seems to reinforce my initial > assumption about the Tx-Rx cavities. Do you agree? > > > SECOND QUESTION TOPIC: > > The other ham thought that I should end up with a band-pass that is > narrow enough to eliminate adjacent repeaters (at 16kHz spacing, as > per the current SCRRBA band-plan separation). I tried but I cannot > get either set of cavities to have that narrow of a band-pass; at > least not without sacraficing most of the signal in the process. I > suspect that the receiver and transmitter need to actually inforce > these much narrower bandwidth requirements within the broader > protection provided by the cavity-duplexer. I suspect that I should > focus my duplexer tuning on passing the desired frequency, notching > the alternate repeater frequency, and trying to do so with the least > amount of signal attenuation. Am I correct in these understandings? > > > > For both of these question topics, feel free to point out anything > that I might be missing or misunderstanding. I'm by no means > sensative about this stuff, and still consider myself on the learning > curve about duplexers and repeaters in general. > > Thank-you (in advance) for your time, thought, and opinions. > > <<< vern >>> >
